Photographic printing



Jan. 19, 1960 D. R. CRAIG 2,921,512

PHOTQGRAPHIC PRINTING Filed June 17, 1957 LOGARlTHMlC A.C. INTEGRATOR NETWORK AMPLIFIER 5f 32 36 Y 62 PEAK 'ro PEAK SPEED DETECTOR REFERENCE SCALE TRIGGERED REFERENCE 58 ,34 52 SWITCH k FIG-.2

PFZIN'I" DENSITY H LOG E XPOSU RE INVENTOR -$PEE0 H DWIN R. came ATTORNEY United States Patent I 2,921,512 PHOTYOGRAPHIC PRINTING DwinR. Craig, Falls Church, Va., assignor to Logetronics, Inc., Alexandria, Va., a corporation of Delaware Application June 17, 1957, Serial No. 666,124

6 Claims. (Cl. 95-73) This invention relates to photographic printing and particularly to methods and apparatus for producing contact prints substantially automatically.

Obtaining a good print from a negative has traditionally been a matter of trial and error, based on the best judgmerit of highly experienced laboratory technicians. It is the purpose of this invention to eliminate completely the need for such judgment, by incorporating in an automatic contact printer not only a means for evaluating negatives but also additional means for properly executing the exposure. These evaluation and execution functions are performed simultaneously bymeans of feedback from a plurality of light sensing devices to a plurality of light sources, by virtue of which it becomes possible for the first time to print all black and white negatives on a single type of conventional high contrast printing material. Since all aspects of the final print are determined during exposure in the contact printer, it is also possible in accordance with the present invention to utilize a standardized developing technique regardless of the character of the negative being printed. The combination of standardized printing material, automatic contact printer, and standardized developing techniques, therefore reduces the production of high quality prints from a variety of negatives to a matter of routine. Exposure of the print is obviously the key to such an operation.

The contact printer contemplated by this invention performs automatically the several functions of dodging, choosing exposure level, and selecting the final print contrast; In quality printing, dodging has always been necessary to overcome the inherently short exposure scale and non-linear reproduction characteristics of printing papers; choice of exposure level has been achieved in the past by selecting an exposure time corresponding to the average density of the negative being printed; and the determination of final print contrast has been effected previously by the selection of a printing material Whose exposure scale would equal that of the negative. Since allof these functions, or their equivalents, still appear to be. necessary in quality printing, it is among the objects of this invention to provide methods and apparatus whereby they will be performed automatically, and by virtue of which their combined effect permits all negatives to be printed on a single emulsion, to be followed by a standardized development.

According to this invention, duration of the exposure is determined by a phototube and integrating circuit, by which light which has penetrated both the negative and photosensitized paper or other carrier will be integrated with respect to time. When the value of integrated light reaches a predetermined level corresponding to the speed of the emulsion, an electronic switch operates to terminate the exposure by extinguishing or otherwise rendering ineffective the exposing light. Consequently, varia tions between negatives and variations in average light level during exposure do not affect final print density.

Autorriatic dodging is performed by using a cathode ray tube as the printing light source, and employing a second phototube for continuously measuring the brightness of the scanning spot after it has penetrated both the negaf tive and the photosensitized paper. Variations in signal level to control brightness of the cathode ray tube are fed back inversely from the phototube so that the scanning spot becomes brighter for denser areas in the negative and dimmer for thinner areas. The amount of inverse feedback applied is a function of density variations within the negative and is automatically controlled to permit residual excursions in brightness at the phototube which correspond to the exposure scale of the printing material. Extreme subject contrast can produce negatives of corresponding extreme contrast which may fall beyond the limits of the dodging circuitry and according to this invention the existence of such a negative is recognized by its peak to peak amplitude in the dodging channel, whereupon the automatic contrast control becomes eifective. This automatic contrast control comprises a second light source arranged to superimpose a uniform layer of non-image forming light on the emulsion of the printing material, which is effected in a contact printer by transmitting light to the emulsion through the paper or other base of the printing material. This combined exposure to image forming light and non-image forming light causes a reduction in contrast similar to that produced by flashing as used in the graphic arts, or similar to the reduction in contrast encountered in aerial photography by the interposition of haze between the ground and the camera.

A more complete understanding of the invention will follow from a description of the accompanying drawings wherein:

Fig. l is a schematic circuit diagram depicting a contemplated form of apparatus; and

Fig. 2 is a curve which will assist an understanding of the principles of the invention.

Referring to Fig. 2, there is depicted a curve showing the H and D characteristics of a typical photographic printing emulsion, and illustrating the combined effect of the three automatic exposure functions contemplated by this invention. As commonly designated in this art, E represents exposure and log E symbolizes the logarithm thereof. The speedof the photosensitive emulsion is defined as the value of log E necessary to produce a middle density in the final print. The exposure scale 7 of the photosensitive print material is defined as the difference in log E which can be reproduced on the substantially linear portion of the D, log E characteristic. As illustrated in Fig. 2, the difference in transmission of the negative between its highlight and shadow areas (S-H) exceeds the scale of the printing material. Consequently, in ordinary printing, both the shadows and highlights would be reproduced on the flat portion of the D, log E curve, highlight contrast H would be reduced to'print contrast H (underexposed), and shadow contrast S would be reduced to print contrast S (overexposed). With automatic dodging however, more light is passed through the dense areas of the negative and less light is passed through its thinareas, producing an apparent compression of the negative which brings the values of S and H closer together on the log E scale, reproducing the highlights in the print as H and the shadows as S Highlight contrast H is here defined as density variation within an area smaller than the scanning spot but in a region of high density within the negative, and shadow contrast as density variation within an area smaller than the spot but in a thin region of the negative. These definitions provide the basis for a distinction between detail contrast and gross contrast, detail contrast referring to the length of either S or H, and gross contrast to their degree of separation. Accordingly, it can be seen that the faceof the tube.

automatic dodging tends to reduce gross contrast, S- H,

while at the same time maximizing detail contrast, observing that S is greater than S and H is greater than H Whereas a scanning spot can exert no expl cit influence on detail smaller than its own dimensions, it is highlight contrast H in the print is still less than highlight contrast in the negative H. This situation represents the case Where gross contrast in the negative ex- 'ceeds that which can be controlled by automatlc dodging.

To correct this situation, the printer must automatically inject some non-image forming light to further reduce contrast in the final print. If we assume that the relative intensity of light passing through S and H is expressed image forming light, the new ratio becomes 11:2, representing a substantial reduction in contrast, as required. The shadow exposure on the other hand, was increased from 10 to 11, representing an increase in this value of only 10%, as compared with an increase of highlight exposure from 1 to 2, or 100%. Consequently, print contrast in the shadows 8;, is relatively unaffected, whereas print contrast in the highlightshas been increased from H to H which in turn is now equal to highlight contrast in thenegative H. From the foregoing it is apparent that a combination of the three functions described is capable of altering the apparent contrast of the negative, both gross and detail, to fit the exposure scale of the printing material. Exposure duration determines average print density, automatic dodging reduces gross contrast and maximum detail contrashand superposition of non-image forminglight further reduces both gross and detail contrast. It is therefore necessary to choose a printing material with sufficiently high gamma to accommodate the fiattest negative which will be encountered in practice, whereupon all other negatives will be reduced to this same apparent contrast. a

As depicted in Fig. l, the paper or other support 10 is coated with an emulsion providing a photosensitive surthe ratio 10:1, by the addition of one unit of non- I predetermined value. During the exposure, the pulsed light and DC. component of light sensed by the photo'- tube 26 are combined to discharge a capacitor constituting a part of the integrator 30. Charge removed from this capacitor corresponds to the number of photons received by the phototube 26 which in turn corresponds to the amount of light incident on the portion of the area of the printing material to which the phototube is exposed. When the voltage on the integrating capacitor has dropped from a fixed preset value to a reference value manually selected by adjustment of the speed reference 36, the electronic switch 34 is triggered. The manually inserted voltage reference corresponds to the speed of the printing material constituting the photosensitive surface 12.' This in turn corresponds to the amount of light which is necessary within thev field of view of the phototube 26 to expose the printing material to a mid-tone of grey, and preferably the field of view of the phototube 26 can be manually directed by the operator to aim at any selected center of interestof the. picture.

This selected region will always receive the optimum exposure and will usually be bracketed by other areas of extreme highlight or shadow. When the electronic switch 34 is triggered by the integrator 30, a signal is transmitted through the lead to the control grid 22 of the cathode ray tube 16 to terminate the exposure.

During the exposure period, a separate phototube 42 is provided in the path of light penetratingthe support 10 to perform the functions of automatic dodging control and automatic contrast control. The signal from thephototube 42 which views the entire format, is passed through a lead 44 to a logarithmic network 46. This network converts relative transmission'of light into the logarithm of relative transmission providing a more convenient notation for'rela'ting subsequent control'functions to the D, log E characteristics of the photographic emulsion;

. it permits the system to cope with large variations in light from its cathode 20 under the control of one or more control electrodes 22. The resulting spot of light at the face of the tube is projected by means of a lens 24 through the negative 14 exposing the photosensitive surface as it scans the area to be printed. The size of the projected spot depends on not only the electronic focus within the tube but also the optical focus and scale of magnifi'cationbetween the face of the tube and the negative. With perfect optics, the ratio of spot diameter to format dimensions will always be the same at the negative as at In practice, this will produce a relatively unsharp dodging pattern as compared with the sharpness of images generally recorded on photographic materials. A spot having a diameter of approximately 4;" covering a format whose dimensions are, say, 8" by 10", can be formed at a brightness level suflicient'to represent practical printing speeds.

'A phototube 26 is exposed to light penetrating the support 10 through a shield 29 containing an aperture 31 which restricts the field of view to a portion onlyof the format. This phototube 26 is D.C. coupled by a lead 28 to an integrator 30 which is connected by a lead 32 to a triggered switch 34 to which a source of adjustable speed reference voltage 36 is also connected by a lead 38. The triggered switch is in turn connected by alead 40 to the control electrode means 22 for terminatmg the exposure when the integrated light achieves a print would be reduced to zero.

level, without producing voltage limiting in the electronic circuitry; and it tends to provide a constant loop gain for signals representing variations within the negative, even though the absolute light level may be altered drastically due to changes in lens aperture, average negative density, or transmission of the printing material.

The signal from the logarithmic network 46 is fed through a lead 48 to an AC. amplifier '50 where it is inverted, amplified, and fed through a lead 52 back to the grid 22 of the cathode ray tube 16. 'Response of this feedback loop is' rapid compared to the velocity of the spot in order to establish instantaneous and continuous control of spot brightness. If this loop were allowed to operate at inverse feedback at which achange in spot brightness exactly matches and'cancels a change in negative transmission, the gross contrast in the final Such prints are highly informative in that they provide a maximum of detail contrast throughout, a real asset to documentary photog- .rap'hy such as aerial and scientific, but they are not pleasing froma pictorial standpoint. To avoid this severe reduction in gross contrast, it is necessary toreduce the value of loop gain'and allow some variation in bright ness'at the phototube 42. r a

.The signal from the logarithmic network 46 is also fed through a lead 54 to a peak to peak detector 56, such peak to peak signal, without feedback, representing the scale of the negative which isrepresented in Fig. 2 as S-H. 'A manually inserted scale or voltage reference 58, connected to the peak to peak detector 56 by a lead 60 corresponds to the exposure scale ofrthe photographic emulsion 12. 'When the peak to peak signal exceeds this reference voltage, a signal is passed along a'lead 62 to the AC. amplifier, increasing its gain from zero to a positive value, which in turn provides inverse feedback via the lead 22 to the control electrode 22 of the cathode tube 16. When, after a few sweeps, this inverse. feedback. has reduced the peak to peak signal. back to the value of the scale reference 58, the gain of the A.C. amplifier is then allowed to-drift back towards zero. Following this, the peak to peak signal recovers, once again introducing inverse feedback to the grid 22. This type of hunting action thus maintains the peak to peak signal at a level corresponding to the scale of the emulsion.

When a negative is encountered whose density scale exceeds that which can be brought within acceptable range by the dodging control, it becomes necessary to introduce suitable contrast control in addition. A second ary light source 64 is arranged to illuminate the format uniformly with actinic light, passing through the back of the paper to the emulsion 12. The brightness of this light is controlled by the value of the peak to peak voltage from the phototube 42 and the logarithmic network 46. An argon glow lamp is suitable for this application since it provides highly actinic emission and is readily controlled by low power electronic circuitry.

A contact printer embodying these features can be combined with mechanical means for positioning negatives and paper automatically so as to be capable of operation without supervision or need for operator decisions. After an initial calibration to establish the references for speed and scale of the printing emulsion, no further attention would be required, even though the negatives to be printed varied as widely as those encountered in ordinary amateur photography.

To illustrate the three exposure functions and their interdependence, it will be appropriate to consider the cause and effect of three general types of negatives which might be encountered. For convenience, the three classes will be designated respectively, thin, normal, and contrasty. The thin negative is usually the result of either underexposure or underdevelopment. If underexposure, it will have low contrast, being reproduced on the toe of the characteristic curve of the negative material. If thin because of underdevelopment, it will likewise exhibit low contrast because of the developing characteristics of films. In either case, both the gross contrast and the detail contrast will be low. Therefore, the phototube 42 will sense very little peak to peak signal and there would be little, if any, inverse feedback for dodging. Furthermore, the peak to peak signal would fail to exceed the scale reference 58 and would produce no light from the secondary source 64 for contrast control, as result of which, the print produced would be characterized by maximum contrast.

The normal negative usually exhibits gross contrast which exceeds that capable of reproduction by high contrast printing materials. Consequently, during exposure the peak to peak signal would exceed the scale reference and would produce inverse feedback for dodging, in which case the amount of dodging would be just sufiicient to prevent either the shadows or highlights from being reproduced on non-linear portions of the curve.

Contrasty negatives are usually the result of subject matter possessing a high degree of contrast, such as those illuminated by bright sunlight or photo flash sources, but they may be the result of overdevelopment of the film. In either case, both the detail contrast and the gross contrast would be extremely high. During the printing operation according to this invention, the peak to peak signal would exceed by far the scale reference tending to produce inverse feedback approaching 100%, a condition which is seldom if ever achieved due to practical limitations. Under these conditions however, the contrast control would be operating to energize the secondary light source 64 and thereby reduce not only the gross contrast but also the detail contrast to a point where the image could be reproduced by a standardized type of photosensitive coated paper made possible by this invention.

Whereas only one form of the invention has been shown and described, it should be construed as illustrative and should not be restrictive beyond the scope of the appended claims.

I claim:

1. Photographic printing apparatus comprising a primary light source for exposing a photosensitive emulsion through a transparency, photoelectric control means in circuit with said source controlling said source as an inverse function of the intensity of light penetrating said emulsion, photoelectric control means in circuit with said source controlling said source as a function of the total light penetrating said emulsion to terminate an exposure cycle, a secondary light source in an optical path including said emulsion but remote from said transparency, and circuit means including said secondary source and the first said photoelectric control means for energizing said secondary source to expose said emulsion substantially uniformly as a' function of the contrast of said transparency.

2. Photographic printing apparatus as set forth in claim 1 wherein said primary light source is a cathode ray tube.

3. Photographic printing apparatus as set forth in claim 1 wherein the first said photoelectric control means includes a phototube in the path of light passing through said transparency, an output circuit for said phototube, a logarithmic network and an A.C. amplifier, said network and amplifier interconnecting said output circuit and source controlling circuit.

4. Photographic printing apparatus as set forth in claim 1 wherein the circuit means including said secondary source and the first said photoelectric control means comprises a phototube in the path of light passing through said transparency, an output circuit for said phototube, a peak to peak detector in said output circuit, an adjustable source of reference voltage connected to said detector, and an output circuit for said detector including said secondary source.

5. Photographic printing apparatus as set forth in claim 1 wherein the second said photoelectric control means includes a circuit containing a phototube, exposed to the light penetrating the emulsion, switching means connected to the light source, and integrating means responding to total light impinging on said phototube to actuate said switching means.

6. Photographic printing apparatus comprising a cathode ray light source having a cathode and control electrode means, support means for a transparency and a photosensitive surface to be exposed in the optical path of light produced by said source, a photoelectric sensing device in the optical path of light to which said surface is exposed, an inverse feedback amplifier and a peak to peak detector in circuit with said sensing device and control electrode means, a second source of light for ex posing said surface in circuit with said detector; and a circuit including said control electrode means, switching means, and integrating means responsive to light impinging on said surface to control the duration of an expose ing cycle.

References Cited in the file of this patent UNITED STATES PATENTS 2,186,942 Vierling Jan. 16, 1940 2,352,914 Rackett July 4, 1944 2,543,706 Pohl Feb. 27, 1951 2,565,399 Simmon Aug. 31, 1951 2,691,917 Curry Oct. 19, 1954 2,783,678 Andreas Mar. 5, 1957 

